1. Field of the Invention
The present relates to novel hydrazone compounds, to processes for the preparation thereof and to electrophotographic photoreceptors comprising them, especially to electrophotographic photoreceptors each of which comprise one of the novel hydrazone compounds as a charge-transporting material in a photosensitive layer on an electrically-conductive base.
2. Description of the Prior Art
Inorganic photosensitive materials such as selenium, cadmium sulfide and zinc oxide have heretofore been used widely as photosensitive materials for electrophotographic photoreceptors. Photoreceptors using these photosensitive materials do not, however, fully provide the properties required of electrophotographic photoreceptors, such as sensitivity, light stability, moisture resistance and durability. For example, although photoreceptors based on a selenium material have excellent sensitivity, they have numerous drawbacks, viz., they are prone to crystallize under heat or to deposit smear or the like and their characteristics hence tend to be deteriorated, they are costly as they are fabricated by vacuum deposition, and they cannot satisfactorily be formed into a belt-like configuration due to their lack of flexibility. Photoreceptors using a cadmium sulfide involve problems of moisture resistance and durability, while those employing zinc oxide have a durability problem.
With a view toward overcoming these drawbacks of photoreceptors which use such inorganic photosensitive materials, various photoreceptors using organic photosensitive materials have been investigated.
Among photoreceptors developed to improve such drawbacks, function-separated photoreceptors in which the charge-generating function and the charge-transporting function are assigned to different materials have attracted interest. Since function-separated photoreceptors permit the selection of a material having one of the above two functions and another material having the remaining function or functions from wide ranges of materials and then to use them in combination, it is possible to fabricate photoreceptors having both high sensitivity and high durability.
Electrophotographic characteristics required for a charge-transporting material include:
(1) A sufficiently high ability to receive charges generated by an associated charge-generating material.
(2) An ability to rapidly transport the charges thus received.
(3) An ability to fully transport charges even in a low electric field, so that residual charges do not remain.
In addition, the charge-transporting material is also required to have high durability so that it can remain stable to the light, heat and the like, to which it is repeatedly exposed as a photoreceptor in the course of the repeating steps of charging, exposure, development and transfer upon copying, so that it can thus provide reproduced pictures having high fidelity to the original and good reproduceability.
A variety of compounds have been proposed as charge-transporting materials. For example, poly-N-vinylcarbazole has been known as a photoconducting material for many years. Photoreceptors using this compound as a charge-transporting material have been used commercially. However, this material itself has poor flexibility, is brittle and therefore tends to develop cracks. Accordingly, it has inferior durability with respect to repeated use. When it is used in combination with a binder to improve its flexibility, another problem arises, viz., the electrophotographic characteristics deteriorate.
On the other hand, low molecular weight compounds generally are not film-formers. Therefore, they are generally mixed with a binder at desired ratios to form photosensitive layers. Many charge-transporting materials based on low molecular weight compounds have been proposed. For example, hydrazone compounds have high sensitivity as charge-transporting materials, including those disclosed by way of example in Japanese Patent Laid-Open Nos. 46761/1980, 52064/1980, 58156/1982 and 58157/1982. However, they have a decomposition problem due to the ozone given off upon corona charging or light and heat instability. Although they have excellent initial performance, low-contrast or high fogging pictures are obtained after repeated use because of a reduction in the charge-holding ability or an accumulated residual potential.
Many other charge-transporting materials have also been proposed. However, there is no charge-transporting material which can fully satisfy the performance required as an electrophotographic photoreceptor in actual use. There is hence an outstanding demand for the development of still better photoreceptors.